Low-cost, high efficiency, electrical energy storage (EES) is needed for the future electric grid which will include more variable energy resources, such as wind and solar. Movement towards predominately low-carbon energy systems requires renewable resources and could be accelerated by integration of carbon capture and storage (CCS) with renewable energy. Currently, substantial penetration of wind and solar resources into the electric power grid is challenged by their intermittency and the timing of generation which can place huge ramping requirements on central utility plants [1], which are also limited in dynamic response capability. Storing the electric energy directly into batteries is one of the most efficient ways to preserve the energy generated from renewable resources, but capacity limitations of conventional batteries are too great at present to economically store enough energy at utility-scales. This talk will discuss employing novel EES systems derived from reversible fuel cell technology as a dispatchable energy resource.

Reversible solid oxide cells (ReSOCs) are an electrochemical energy technology capable of providing high roundtrip efficiency and cost-effective electrical energy storage, and offer many advantages over other options. ReSOC systems operate sequentially between fuel-producing electrolysis and power-producing fuel-cell modes with storage of reactants and products (CO₂/CH₄ gases) in tanks for smaller-scale applications (see Fig. 1) and in caverns for larger, seasonal storage [3]. Maintaining the high conversion efficiencies seen in laboratory-scale cell tests at the system-level, requires careful system design to integrate storage and electrochemical conversion functions. By leveraging H-C-O reaction chemistry and operating at intermediate temperature, the ReSOC is mildly exothermic in both operating modes, which simplifies balance-of-plant integration and thermal management. In this talk we discuss the thermodynamics of cell operation and system concepts for leveraging C-H-O chemistry for energy storage and Power-to-Gas production options. System concepts and strategies for effective thermal management and balance-of-plant systems integration, which are critical to achieving high roundtrip efficiencies, are highlighted, including distributed energy systems. The roundtrip efficiency, energy density, and capital cost tradeoffs of these configurations are quantified through computational modeling and production cost estimates for the technology. Because the dynamic response of renewably powered electrolyzer systems is important for meeting grid-energy ramping requirements, transient response characteristics of ReSOC stacks are also summarized. Results indicate that roundtrip efficiency and cost of storage for large-scale energy storage ReSOC systems are about 72% and range from 3-8 ¢/kWh, respectively. Market arbitrage scenarios for large, grid-energy storage could enable storage costs to be converted to revenues. At the distributed scale, system performance of nearly 70% roundtrip efficiency and cost of storage at 6-7 ¢/kWh are possible. The presentation concludes with challenges for technology deployment.

References:

1. CASIO (2013), What the Duck Curve Tells us about Managing a Green Grid, Fast Facts.

2. Wendel and Braun, Applied Energy 172 (2016) 118–131.

3. Jensen et al., Energy Environ. Sci., (2015), 8, 2471.